Concrete Compressive Strength - Existing Bldg Renovation 1

[steellion]

I am working on a project where we intend on keeping the existing CIP concrete structure and retrofitting as necessary to comply with IBC and UFC codes. When existing concrete is being used, is it appropriate to use the current compressive strength (determined from core samples) in the calculations for capacity? All of the ACI code equations typically use f'c, defined as the 28-day compressive strength of the concrete.

This building is 60-70 years old, what if f'c at 28 days is not known?

 

[MiketheEngineer]

Typically Fc will be higher than at 28 days. Why not just do a few tests as you have done and de-rate by 10-20% or to the lower typical values - say 2,000 or 3,000

 

[Ron]

Taking cores and doing compressive strength testing will tell you roughly, what class of concrete you're working with.

Depending on the age of the structure, you can expect the in-place strength represented by cores to be about 15% higher than the core strength. You then can figure backward to see roughly what the class of concrete was at original construction. Generally, you are likely to see the following:

f'c=3000 psi......core strengths from 3500 to 5500 psi
f'c=4000 psi......core strengths from 5000 to 7000 psi
f'c=5000 psi......core strengths from 6000 to 7500 psi

These would be in structures ranging in age from 5 years to over 50 years.

Keep in mind that ready-mix concrete strengths are generally higher than f'c, even at an early age, so as they get older, they will gain strength accordingly. As an example, we see new concrete with f'c=3000 psi being 3500 to 4000 psi at 28 days. From there, it can gain another 20 to 30 percent over many years.

 

[steellion]

Thanks, Ron. So I should still back-figure the original f'c and use that in the calculations? I don't want to penalize myself too much if the average core sample is 6500 psi and it's supposed to be 4000. A 15% reduction is reasonable.

I have done an existing building assessment, and from visual inspection, the structure is in very good shape... almost no spalling or rust damage. I was planning to also include a 5-10% reduction in strength for minor spalling, core holes, etc.

 

[Ron]

steellion..you can use a much of the existing strength as you feel comfortable with. You don't have to go all the way back to an assumed f'c. Consider a group of tests, take the lower if the variation is not great, then discount that some reasonable amount (15-20 percent). You'll still likely be much higher than the original f'c, but that's OK.